A multipoint lock is one that, with a single turn of the key, secures the door at several points at once: as well as the central bolt, it drives rods or bolts upwards, downwards or sideways into the frame. Almost every security door has one.
Spreading the locking across several points makes it far harder to lever the door open, because there is no single spot to get a tool into. It also helps the door leaf pull in tight against the frame and fit better.
If you notice the key going stiff on the last turn, it usually means one of the points is not going straight into its strike plate. It is worth adjusting before a part inside the case breaks. We explain it under multipoint locks.